Open gear lubricating oils and greases are used in machinery employing large, slow-moving gears under heavy load. As the name implies, the gears are open to the atmosphere. Open gear lubricants are subject to particularly severe operating conditions. Not only must the lubricant perform its basic function of minimizing friction and metal-to-metal contact between moving surfaces, but it must also be able operate over a wide temperature range and under high load conditions.
A basic requirement for an open gear lubricant is mechanical shear stability. Shear stability is a measure of the ability of an oil to resist permanent viscosity loss under high shear; the more shear stable an oil, the smaller the viscosity loss when subjected to shear. If the viscosity of the lubricant drops too much during operation, the gears will not be sufficiently lubricated and operators will not know when such a situation will occur. If the viscosity of the lubricant trends upward in a controlled manner during operation, operators will be able to notice the thickening through, e.g., channeling in the gear box and correct the situation with minimal adverse effects to the gears.
Highly adhesive lubricants are required for most open gear applications. Typically, such lubricants are heavy oils, asphalt-based compounds, or soft greases. As refiners turn from solvent refining to newer processes, the availability of heavy oils such as heavy cylinder stock is diminishing. High viscosity synthetic poly-alpha-olefins (PAOs) produced from C8 to C12 linear alpha-olefins are available. These PAOs have good shear stability but are expensive because of the high cost of the linear alpha-olefin raw material. High viscosity polyisobutylenes (PIBs) can also be used as a heavy oil alternative.
High viscosity base stocks can be blended with lower viscosity base stocks to increase viscosities of the low viscosity stocks. There is a need providing open gear lubricants with good shear stability without having to use highly expensive PAOs.